This invention relates to weather radar systems for obtaining rain information by radiating microwave pulses from a radar transmitter and receiving echo signals from weather targets at a radar receiver, and, in particular, to rejection of echo signals from fixed targets such as the ground (which are hereinafter referred to as "ground clutter") included in the received signals.
In weather radar systems, microwave pulses are radiated with a certain repetition frequency from the transmitter and echo signals from weather targets are obtained from the waves received in the receiver by envelope-detection of the received waves. The echo signals are averaged over a number of pulses to obtain the average intensity of the weather echo and applied to an indicator such as a plan position indicator (PPI), an A-scope indicator and others to display the geometric pattern and motion of weather targets such as rain, clouds, storms and so on. Since the amplitude of the averaged signal corresponds to the average intensity of the weather echo which depends on, for example, the rain amount, the rain amount at a certain distance from the radar can be seen from the brightness of the pattern on the indicator.
One of the difficult problems in weather radar systems is rejection of ground clutter. The reflected wave received at the radar receiver includes echoes from fixed objects such as mountains, buildings and other objects on the ground (this echo is referred to as "ground clutter"), besides weather echoes. The ground clutter intrudes in echo signals of the weather target to make it difficult to obtain the weather pattern and other information from the echo signals.
Shimizu and Goto, two of the inventors of this invention already proposed an automatic ground lutter rejection system in U.S. Pat. No. 4,139,847. According to the method proposed in the U.S. patent, the received wave in the weather radar receiver is linear-detected and each one of successive pulses obtained by the linear-detection is compared with another later one which is spaced apart from each echo signal by a time period that the fluctuation of weather echo signals is independent to provide difference signals as signals corresponding to fluctuation of the weather target. The variance of the difference signal is proportional to the mean power of the weather echo. Therefore, the difference signals give signals corresponding to the average intensity of the weather echo wherein the ground clutter is effectively eliminated. The difference signals are unipolarized, or squared, and thereafter, are averaged. The averaged signals have an amplitude corresponding to the average intensity of the weather echo. Therefore, the display of the weather pattern, the observation of the rain amount and other analysis can be readily and accurately made by employing the averaged signals in the known technique and devices.
The ground clutter rejection proposed in the aforesaid U.S. patent is based on a provision that the intensity of the ground clutter is constant over the time period between two compared echo signals. But, the intensity of the ground clutter actually varies because, for example, the dielectric constant of fixed targets such as mountains varies and because trees move. Therefore, the difference signals from the comparing circuit includes not only the signals corresponding to the fluctuation of the weather target but also the signals corresponding to the fluctuation of the ground clutter. This means that the average intensity of the weather echo is erroneously observed greater. In the range having greater ground clutter, the weather observation is actually difficult using the weather radar system.